The present invention relates to a method for continuously producing strip steel or steel sheet from flat stock produced in accordance with the arcuate continuous casting method with a horizontal direction of delivery.
The steel industry faces a great need, either as the result of a general trend or in order to overcome the crisis with which in recent years particularly the operators of out-dated systems have been confronted, to lower operating and investment costs while, at the same time, improving product quality and increasing flexibility with respect to the lots produced, i.e., so-called "coils" or steel sheets. With the modernization of existing steel mills or the planning and creation of new steel mills with the use of new technological concepts and devices, a paramount goal has been to increase productivity and profitability while simultaneously improving product quality and achieving a greater range with regard to the "unit size" in which the final product is to be delivered in order to cover the largest possible range of uses.
One of the new technologies that is currently being promoted in view of the current demands on steel production involves the processing steps between a melting of steel and either (i) the winding up of strip steel in the form of "coils" or (ii) the stacking of sheets. The new technology comprises the casting of thin slabs in a thickness close to their final dimensions, which can then be processed further into the desired final product in only a few subsequent passes or deformation steps. This has led to remarkable improvements in continuous casting technology, particularly with respect to mold construction and of the corresponding immersed outlet, and also to improvements in the construction of roll stands and trains with the goal of achieving the desired deformation in the fewest possible number of passes.
Plants for the production of strip steel have become known, even though described as "pilot plants", in which thin slabs of a thickness of about 50 mm are produced by the continuous casting method, as compared with conventional slabs having a thickness range of 150 to 320 mm. These thin slabs pass through the successive rolling/processing steps in different ways; the final product is strip steel of a thickness of only a few millimeters. It has been previously proposed to roll out the cast product, immediately after an intermediate heating in a furnace, for instance in a tandem train having six stands. Since the casting speed cannot be much faster than about 5 meters per minute, the rolling speeds which thus results on the last stand of the rolling train are too slow to maintain the required final rolling temperatures of at least about 865.degree. C. That is to say, the strip undergoes excessive cooling between one rolling step and the next step due to its low speed, which is identical to the casting speed, upon entering the rolling train. This solution was, therefore, abandoned since it was not possible to solve the problem economically even with heat protection devices and heated rolls because this would have resulted in a considerable increase in investment and operating expenses.
Another approach proposes cutting the strip in front of a heating furnace in which the heat treatment (temperature equalization) of the strip over its entire cross-section subsequently takes place. It can, for instance, be a gas-heated roller furnace with which, independent of the casting speed which is to be taken into account, a temperature of the strip at the outlet of the furnace of about 1100.degree. C. can be set, i.e. a temperature which is optimal for the subsequent rolling process. The strip is cut to a standard length, which for a certain weight of the coil can, for instance, be about 50 meters, which requires a corresponding furnace length of about 150 meters if the required buffer action is taken into account.
By uncoupling the rolling train from the casting process proper, the rolling out of the thin slab or "rough strip" can be carried out at higher speeds, so that a drop in temperature to below the minimum temperature permissible for the final rolling stage need not be feared. In this connection, the length of the furnace--which amounts to about three times the length of the length of strip--results, aside from a considerable increase in equipment investment, also in enormous space requirements which cannot be satisfied by many steel mills.
In addition, the dimensions of the plant and thus of the furnace impose limits on the length of the successive lengths of strip to be treated and thus also on the final weight of the coil. The coil's final weight, in turn, limits the range of use for production of coils of very large diameter. Accordingly, a plant of this type also does not afford the possibility of using even thinner initial slabs should this become possible as a result of the further technological development of the continuous casting method. Assuming an initial thickness of 25 mm--as has already been done hypothetically--instead of 50 mm, the strip would have to be divided into lengths of about 100 meters in order to obtain the same final weight of the coil, which would require a length on the order of magnitude of about 300 meters for the treatment furnace, which is not feasible both from a practical and from an economic standpoint.
It is, therefore, an object of the present invention to create a method of the type described above and a corresponding plant to carry out this method by means of which a steel strip can be continuously produced from a flat product coming from an arcuate continuous casting plant without incurring the above-mentioned disadvantages.
In particular, cutting of the strand between the casting and at least the first rolling is dispensed with, the casting strand passing in the first roll stand at the speed at which the rolled stock leaves the arcuate section of the continuous casting plant. Thus, the method is to be carried out "in line" with practically unlimited flexibility so that it becomes possible to produce coils of any desired weight and length or sheets without changing the dimension parameters of the plant since the cutting of the rolled strip is conducted at least after the first rolling or after conducting all operating steps directly in front of the reeling or stacking device.
The foregoing object is achieved by a method which is characterized by the following steps:
a) Shaping flat stock after complete solidification of the strand in a first shaping step at temperatures of more than 1100.degree. C.; PA1 b) Inductive reheating of the flat stock to a temperature of about 100.degree. C. with approximate temperature equalization over the entire cross-section of the flat stock; and PA1 c) Shaping of the flat stock in at least one additional shaping step at rolling speeds corresponding to the specific reduction per pass. PA1 Adjustment of the shaping steps after passage of a starting bar which is provided upon the casting process; PA1 Separation of the starting bar directly prior to winding up the strip or prior to stacking of the sheets; and PA1 Differentiated heat control in successive steps/zones after passage of the initial bar. PA1 a) A mold for the continuous casting of flat products with a subsequent guide stand in arcuate shape; PA1 b) A first shaping unit for forming the flat product in the guide stand and/or immediately behind it; PA1 c) A device for inductive heating and for approximate temperature equalization over the cross-section of the flat product; PA1 d) At least one additional roll stand; and PA1 e) A cutting device.
In a further embodiment of the method, the strip between the first and the next deformation step is wound up. The rolled-out strip can be wound up according to the desired weight of coil following the forming of the flat product, or it can be stacked after cutting the rolled-out strip following the forming of the flat product in predetermined lengths so as to form stacks of steel sheet, possibly after cooling and straightening. The flat product is therefore first of all passed through a first roll stand at the speed of emergence of the product from the arcuate continuous casting plant and passes through the successive rolling stages always at speeds which correspond to the deformations in the individual passes. The strip which has been rolled in this manner is then either wound up and cut when the desired weight of coil is reached or the strip is subdivided into desired lengths and stacked as sheets. An important aspect of the present invention is the inductive reheating of the flat product, after descaling, to temperatures of about 1100.degree. C. preferably with the best possible temperature equalization since, in this way, excessive cooling of the strip can be favorably counteracted.
A further embodiment of the invention includes one or more steps of inductive intermediate heating of the flat product between the above-mentioned shaping steps. By this intermediate heating, excessive cooling of the rolling stock is also counteracted, so that the required roll temperatures can always be set in the manner that the temperatures in the last shaping step do not drop below a limit value of 860.degree. C.
Another embodiment of the invention provides the following additional steps:
After passage through the roll stands, the starting bar can be cut off by the device which is in any event present for the subdividing of the rolled strips, or it can be cut off by an additional cutting device arranged behind the first forming step.
The plant for carrying out the method of the invention is characterized by the following plant parts in the sequence indicated:
The cutting device can be arranged subsequent to the first shaping unit and between the first shaping unit and the additional shaping unit. A unit can be provided for winding up and unwinding the flat product, the cutting device being arranged in front thereof. The unit for winding up and unwinding the flat product is preferably arranged behind the device for inductive heating and in front of the additional shaping unit.
As an alternative, this system is followed, in accordance with the invention, either by a cutting device for the rolled strip and at least one reel for winding up the strip or by a cutting device for the rolled strip, a cooling device, a straightening machine, and a stacking device for the separated sheets.
In a further embodiment of the invention, the system includes, in addition, at least one inductive heating device in order to effect an intermediate heating between the additional roll stands.
Each of these devices is advantageously provided with heating stages that can be individually controlled.
In accordance with an embodiment of the invention, the system is furthermore equipped with devices for adjusting the cross-section of passage between the rollers of the first shaping unit and the additional roll stands in order to permit the passage of the starting bar present at the head of the casting strand and to reduce the cross-sections back to the customary passage values immediately after passage of the starting strand. Devices are provided for successive control of the individual heating stages of the furnaces immediately after passage of the starting bar. The cutting device for cutting off the starting bar is also used for cutting the rolled strip, i.e. as the cutting device present in the final section of the plant.
In accordance with another proposal of the invention, the cutting device that is arranged behind the first shaping unit is used for cutting off the starting bar.